The difference in mileage sounds right. It doesn't matter what you tow with, you can expect between 9 and 11 mpg. Mostly, right at 10 mph for "most" folks. And it doesn't matter what tow vehicle you have, what brand, or what size engine. They all round out, near 9-10 mpg when towing ....
Why the difference? It's because the trailer creates a gigantic "sail" at the back of your tow vehicle. Literally, a solid wall blowing into the wind. That's a LOT of wind resistance. Add to that the weight of your camper and effect on fuel mileage is horrendious.
Everything above, around 50 mph will start reducing your fuel mileage drastically, simply because of the wind resistance against your camper. At 55 mph, you may get 12 miles a gallon. At 65 you may get 9.5. At 75 you may get 8 mpg. It's all because of the wind resistance against the trailer. The faster you drive, the greater the resistance, the less the mileage.
What you need to do is find your specific rig's "sweet spot". What is the fastest speed you can go for the most effecient fuel mileage. That may be 45 mph to get 14 mpg, or it may be 55 mph to get 10 mpg. Anything faster or slower results in even less mpg. It takes time, patients, and a lot of fuel ups and distance driving to figure it out.
The other thing that grossly affects your fuel mileage when towing is how often you come to a complete dead stop and then start moving again. Stop signs and stop lights in traffic. Any time the rig comes to a complete stop, the engine has to overcome static inertia (a dead stop). It takes an incredible amount of fuel to start moving forward from a dead stop. So, the heavier traffic you are in, the worse your fuel mileage will be also.
Another fuel hog are jack-rabbit starts or sudden fast increases of speed. Every time you push the peddle down for more speed, the amount of fuel used to increase the speed is astronomical. When towing, the most effective way to stretch fuel mileage is to increase speed slowly. You simply cannot drive towing a trailer the same way you drive an automobile when not towing. Oh.... you can, yes ... but it WILL kill your fuel mileasge.
So, bottom line, before trading vehicles, experiment with your driving speeds.
Bottom line:
1) slower speeds means greater fuel mileage, but takes longer to reach the destination.
2) faster speeds means lousy fuel mileage, but reaching the destination is much quicker.
You have to determine which is more important, stretching the fuel mileage and saving the cost, but taking longer to get "there." Or blowing a wad of money on fuel, but getting "there" fast!